Many anthropogenic stressors broadly inflict mortality or reduce fecundity, including habitat destruction, pollution, climate change,invasive species, and multispecies harvesting. Here, we show—in four analytical models ofinterspecies competition—that broadlyinflicted stressors disproportionately cause competitive exclusions within groups of ecologically similar species. As a result, wepredict that ecosystems become progressivelythinner—that is, they have progressively less functional redundancy—as broadlyinflicted stressors become progressively more intense. This may negatively affect the temporal stability of ecosystem functions, butit also buffers ecosystem productivity against stress by favoring species less sensitive to the stressors. Our main result follows fromthe weak limiting similarity principle: species with more similar ecological niches compete more strongly, and their coexistence canbe upset by smaller perturbations. We show that stressors can cause indirect competitive exclusions at much lower stressor intensitythan needed to directly cause species extinction, consistent with the finding of empirical studies that species interactions are often theproximal drivers of local extinctions. The excluded species are more sensitive to the stressor relative to their ecologically similarcompetitors. Moreover, broadly inflicted stressors may cause hydra effects—where higher stressor intensity results in higherabundance for a species with lower sensitivity to the stressor than its competitors. Correlations between stressor impacts andecological niches reduce the potential for indirect competitive exclusions, but they consequently also reducethe buffering effectof ecosystem thinning on ecosystem productivity. Our findings suggest that ecosystems experiencing stress may continue toprovision ecosystem services but lose functional redundancy and stability.